[Django]-Display multiple related Django models in a table design problem

If you have that date model so you can fetch all bs then group them by a and calculate total amount. Code may look like this:

View:

from django.utils.itercompat import groupby

def view(request):
   bs = B.objects.all().annotate(sum_of_c_val=Sum('c.val'))\
                       .select_related('a')
   b_by_a = [
            {
               'grouper': key,
               'list': list(val),
               'total': sum([b.sum_of_c_val for b in val])
            }
            for key, val in
            groupby(bs, lambda b: b.a)
   ]

   return render_to_response('tmpl.html', {'b_by_a': b_by_a})

And template:

{% for b_group in b_by_a %}
  {% for b in b_group.list %}
    <tr>
        <td>{{b_group.grouper.name}}</td>
        <td>{{b.name}}</td>
        <td>{{b.sum_of_c_val}}</td>
    </tr>
  {% endfor %}
  <tr>
      <td>{{b_group.grouper.name}}</td>
      <td>Total</td>
      <td>{{b_group.total}}</td>
  </tr>
{% endfor %}

EDIT:

For compatible with Django 1.0 you have to replace annotate call with extra select or in-python amount calculation.

1. With subquery:

   bs = B.objects.all().extra(
       select={
         'sum_of_c_val': 'SELECT SUM(app_c.val) FROM app_c WHERE app_c.b_id=app_b.id'
       }).select_related('a')
   #...
   

   Where app – application name

2. This in-python calculation:

    bs = B.objects.all().select_related('a')
    for b in bs:
        b.sum_of_c_val = sum(map(int, b.c.all().values_list("val", flat=True)))
    #...
   

   But it produces N additional queries (where N = len(bs))